"Its 2:30am in London and I was just woken up with great news. Welcome to the family #20!!" Arison tweeted.

Allen can't formally sign with the Heat until July 11, according to NBA rules.

But the 10-time All-Star had met with Miami officials on Thursday and was reported by US media to have agreed to an offer worth considerably less than what the Celtics were prepared to pay.

Allen spent five seasons with the Celtics, winning an NBA title in 2008.

In June of last year, he exercised his contract option to remain with Boston, keeping intact the team's "Big Three" unit of Allen, Paul Pierce and Kevin Garnett.

This week, however, he has apparently been swayed by Heat president Pat Riley as well as Miami stars LeBron James and Dwyane Wade into making the move south and trying to help the Heat back up the NBA title they won last month with a finals triumph over the Oklahoma City Thunder.

James had encouraged Allen via Twitter, tweeting a picture of Allen in a Heat jersey with the caption "This looks damn good to me."